Varactor diodes (referred to herein as “varactors”) are well known in the art. Such components serve a myriad of purposes. As one illustration in this regard, varactors are sometimes used to facilitate the provision of a voltage controlled oscillator. For example, high-Q varactor diodes can be used to embody a voltage controlled oscillator that is effective, in turn, to provide sufficient synthesizer tuning range and spectral purity of a high performance frequency synthesizer as might be used in a modern multi-band two-way wireless platform.
There are certain problems with such an approach, however. As one example in this regard, integrated circuit manufacturers are continuing to move towards advanced low voltage processes. 1.8 volt processes, for example, represent a particularly coveted design requirement, goal, and opportunity at present. The aforementioned use of varactors, however, seems to conflict with such a trend. In particular, a typical approach to achieving the desired synthesizer tuning range and spectral purity specifies use of high-Q varactors that are driven by steering line output excursions equal to or exceeding ten volts. Such an operational requirement, of course, seems utterly unachievable using a low voltage process such as the aforementioned 1.8 volt process.
As one approach to resolving this dilemma, one might move the synthesizer charge pump off of the integrated circuit that provides the transceiver functionality of a corresponding wireless platform. Such an approach, however, leaves much to be desired. The charge pump, comprising a relatively complicated circuit in and of itself, would require implementation as a separate integrated circuit in order to approach commercial acceptability. This separate integrated circuit would typically require a relatively high voltage process as noted above. Unfortunately, fabrication facilities are increasingly moving to lower voltage processes. This, in turn, raises the very real problem of being unable to reliably source such a part. Furthermore, such an approach does nothing to address another point of concern in such an application setting; power consumption. These high voltage architectures tend to represent a significant load and hence result in relatively high charge pump power dissipation.
Another approach found in the prior art to attempt to achieve the required tuning range is to selectively apply shunt capacitance across the varactor to increase the total capacitance value. This approach, however, also tends to reduce the rate of change of reactance versus control voltage. This, in turn, tends to prompt the undesirable effect of reducing tuning sensitivity.